In the transformer coil art, a number of methods have been adopted for holding or anchoring the turns of an electrical coil, so that they can resist movement when the turns are subjected to the flow of current and consequent electromagnetic forces tending to move them out of position. A commonly practiced method is to bond the turns of the coil to the layer insulation by the use of resinous adhesive layers or resin solution impregnated layer insulation. This method has not been entirely satisfactory, since by filling the coil with such continuous adhesive layers or completely resin solution impregnated materials, it is rendered impervious to the penetration of oil which is essential in providing high impulse strength in transformers.
To provide improved porous solidification of transformer coils, which will withstand large surges of power with resulting high mechanical stresses, Ford, in U.S. Pat. Nos. 3,237,136 and 3,246,271 has used discontinuously patterned resin solution impregnated kraft paper as the restraint. This method however averages only about a 0.25 mil (0.006mm) to 1.5 mil (0.03mm) adhesive thickness build. This low range can be inadequate in many instances for complete wire to paper bonding, providing insufficient short circuit strength. This impregnation with resin solution saturates the paper fibers under the adhesive pattern. When the patterned paper is subjected to a high humidity atmosphere, the paper surrounding the adhesive pattern can swell such that, in some cases, the adhesive pattern forms a depression and is rendered ineffective to bond coils.
Other methods of coating paper have provided thicker builds of patterned particles using dry powder or fiber application. Uhrig, U.S. Pat. No. 3,671,284; Brehm, U.S. Pat. No. 3,613,635; Meston, U.S. Pat. No. 3,174,328; and Bayer, U.S. Pat. No. 3,557.691 coat paper in a predetermined pattern. Uhrig applies a pattern of adhesive points using a dry resin powder and a hot roll applicator having suitable projection points on its surface. Brehm applies dry resin powder in the form of uniformly distributed dots, using a magnetic hopper application means and a perforated hollow roller.
Meston teaches applying discontinuous dry rayon fiber patterns on a discontinuous adhesive solution precoated moving paper sheet, using a diamond cut out pattern, in a continuous electrostatic coating method. The fibers are blown up past a positive electrode, through a mask moving parallel with and at the same speed as the paper and onto the paper which passes next to and beneath a negative electrode. Bayer coats untreated paper strips with a discontinuous dry resin powder pattern by using a stencil or cut out mask in a continuous electrostatic coating method. The powder passes through the stencil and attaches to the paper which is disposed in the air space between the stencil and a negative plate. None of these methods would appear to provide enough bonding points on both sides of the sheet material to insure adequate bonding between adjacent layers and sufficient bond strength for transformer coil layer insulation application.
What is needed then, is a method of making a highly porous yet completely bonded electrical coil, using layer insulation having a discontinuous, minimum, 0.25 mil (0.006mm), and preferably 0.5 mil to 25 mil (0.013mm to 0.635mm) thick, dry coating pattern of adhesive particles; applied in a manner to allow oil permeation through a plurality of laminated layers after the resin adhesive is set.